1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic apparatus, a printer or the like and to an image forming apparatus having a developing unit which is equipped with developers for plurality of colors and provided rotatably.
2. Description of the Related Art
As an image forming apparatus such as a color copy machine, a color printer or the like which provides color images by an electrophotographic system, there is an apparatus having a revolver-type developing device in which a developing unit having an integrated developing device and toner cartridge and equipped with a plurality of developers is rotated to move a developer of a desired color to a developing position and develop an electrostatic latent image. In conventional such a revolver-type developing unit is driven by one motor in most cases. In such an image forming apparatus, since a desired developer of the heavy-weight developing unit is stopped accurately at a developing position, a motor of high performance is required for driving the developing unit. However, when such a heavy developing unit is to be driven by one motor, a large load torque is generated at a part where the developing unit engages with the motor. This makes the rotational driving of the developing unit unstable, and causes the developing unit to vibrate and deviate from the stopping position. Moreover, there is a risk of step-out of the motor.
Thus, conventional, JP-A-11-316479 discloses an image forming apparatus in which the rotation speed of the revolver unit is adjusted in accordance with periodic change of torque generated between the revolver unit and the motor, thus restraining increase in load torque and preventing the motor from stepping out. However, in this conventional image forming apparatus, though increase in load torque due to the torque change can be restrained, the original load torque itself generated in the gear of the heavy revolver unit is not reduced.
Therefore, the load torque generated in the gear of the revolver unit is still large and it makes rotational driving of the revolver unit unstable. There still is a risk of vibration of the revolver unit and occurrence of step-out of the motor. Particularly, when the acceleration of the motor changes, such as when the rotation of the revolver unit starts or stops, the momentary load torque generated in the gear of the revolver unit increases further. Also, because of the high-speed rotation of the revolver unit due to the higher speed of the recent color apparatus, the load torque generated in the gear of the revolver unit increase further and it may affect vibration of the revolver unit or step-out of the motor.
Thus, it is desired that a highly reliable image forming apparatus should be provided which provides a color developed image by using the revolver-type developing unit, the load torque itself generated in the gear when rotationally driving the developing unit is reduced to prevent vibration of the developing unit or its deviation from the stopping position and to prevent occurrence of step-out of the motor.